A variety of methods and systems are known for the tracking of inventory, raw materials, products, or any other items. Accurate locating, tracking and inventorying of the items is a necessity for operations like manufacturing, warehousing, segregation of defective items, assembling, and for a number of similar operations. A desirable feature in such applications is to determine the physical location of the items, to determine the quantity of the specific item, or perform other similar functions, in the shortest possible time.
Electronic Article Surveillance (EAS) systems or Proximity Detection Systems identify the presence of articles using identification means, or tags, placed on/in an article. Such systems are typically used in retail, manufacturing units, packaging department, grocery, library, and the like. Radio frequency identification (RFID) tags and barcode labels are some of the technologies used for EAS systems.
Barcode labels have been used in monitoring inventory. These barcodes are scanned in order to track articles. In order to read the barcode, the barcode label has to come in direct contact with or in close proximity of an optical scanner/reader and therefore cannot be read remotely. Also, barcode labels cannot be used as security devices.
Tags can be broadly classified as read/write and read only. The data stored on read/write tags can be edited, added to, or completely rewritten, but only if the tag is within the range of the reader. The data stored on a read-only tag can be read, but cannot be edited in any manner. Tags used for barcode systems are read-only whereas tags used for RFID are not restricted thereto and may also be electronic read/write devices. The data transmitted by the tag may provide identification or location information, or specifics about the item being tagged, such as price, color, date of purchase, etc. The tag information may be incorporated in accordance with the design of the application or the user's specifications. Information stored in the tag is extracted by electronically interrogating the tag either by physical contact or by remote sensing.
The barcode system suffers from several drawbacks, such as the necessity to have a line of sight, typically several centimeters, between the barcode label/tag and the barcode scanner. Also, items with the printed barcode labels may get damaged due to improper handling making the label or tag unreadable. Barcodes may also suffer from potential problems associated with substandard print quality of barcode labels, which in turn can lead to scanning and reading problems.
RFID tags are attached to items to be monitored. RFID tags are typically small devices that can be embedded in, or attached to, objects for the purpose of identifying the object over a radio channel. RFID tags can be thought of as “digital barcodes”, with the advantage that objects tagged with RFID technology can be more easily and more frequently read compared to, for example, barcode labels, thus improving the quality of information on objects in a supply chain or in the inventory of a warehouse.
Each RFID tag has a unique identification number for the item. Depending upon the type of tag, the reader can receive detailed information stored on the tag, for example, in the memory of the tag, or retrieved from a back-end database using the identification number of the tag as the key to that back-end database. An RFID reader can receive data from as many as 100 tags per second. Accordingly, an inventory can be taken in a time of reduced duration compared to previously-proposed identification systems, such as, for example, the barcode system. In RFID systems, human interference in the reading process is reduced and hence the human error involved is also reduced.
The purpose of RFID technology is to tag individual items so that the item can be monitored according to business requirements. The efficiency of RFID technology directly depends on the quantity and accuracy of the tag ID information collected by readers. However, there are some problems associated with the reading of the tags. For example, RFID tags at the outer periphery of a reader's range are typically more difficult to read due to weaker signal strength(s) compared to those tags that are within that periphery. Furthermore, in this case, the signal strength cannot be boosted and/or the signal range is not extendable by virtue of the tag and/or reader having an associated imposed power supply restriction. In general, there are sources of errors such as defective tags, RF-interferences such as in multiple reader environments, RF-noise, RF-absorptive material, limited functionality due to cost pressure, etc. In general, 100% read accuracy cannot be assured.
In many applications, it is desired not only to monitor individual items/products but also to monitor groups of items/products, such as a pallet of goods leaving a distribution center or arriving at a retail outlet. This is addressed by utilizing “Group Tags”. Since the ability to read a group tag is not assured the identity of individual tags within a group is lost whenever the corresponding group tag is not readable.
To address this problem, it has been previously-proposed that the information about the group members is stored either in a database or on an additional group tag, which is mounted on the case or pallet containing a collection of similar items that are grouped together. In the case of a back-end database, each tag identifier must point to the group information. Alternatively, the distinct group tag identifier may be used as a key to ascertain that all the items associated with the group are present, the tag identifiers that are read have to be cross checked with that database maintained at the point where the item originated. Without the help of a back-end database, all the details of each individual item in the specified group should be stored on the group tag.
The above discussed techniques, however, suffer from certain drawbacks. First, the database should be accessible by a network connection. Secondly, the group tag constitutes a single point of failure—if the group tag cannot be read due to some damage or defective configuration, all information about the group is lost or may not accessible. In such cases, each individual member of the group would have to be read and identified in order to determine or reconstruct the group. This process is expensive and may even be impossible in some cases.
US Patent No. 2004/0046642A1 titled “Protocol for addressing groups of RFID tags” discloses a method of addressing a group of RFID tags, wherein the group of RFID tags comprises a subset of a plurality of RFID tags capable of being addressed by a tag reader, wherein each of the RFID tags in the plurality of RFID tags has a unique identifier, and wherein the method comprises: inserting a group address into a message, wherein the group address comprises a first set of data elements substantially equal in value to corresponding ones of the data elements in the identifiers of the group of RFID tags, and wherein the group address comprises a second set of data elements representing any value for corresponding ones of the data elements in the identifiers of the group of RFID tags, and transmitting the message to the plurality of RFID tags. This method proves helpful in handling group tags, but lacks counter measurement techniques when the group tags are damaged or disrupted.
Accordingly, it is desirable to provide an improved method and system for tag identification, it being also desirable to determine the group completeness of RFID tags without using a group tag and to successfully resolve problems relating to broken links or missing tags. It is yet further desirable to be able to check whether an RFID tag reader has read all items of a case or pallet, i.e. perform a completeness check, without using a group tag, and, if some tags were not reachable or present, to determine the identifiers of these missing tags, without using information in a back-end database, i.e. off-line operation.